In the end of 1960s Prof. Vladimir Zakharov proposed a Wave Turbulence Theory, describing exchange of conserved quantities, e.g. energy between different scales in a weakly nonlinear system. One of the most important applications of this theory is the description of gravity waves in a lake, sea or ocean (relatively long waves, for which capillarity can be neglected). For gravity waves it was shown, that there exists an inverse cascade of wave action, namely very long waves "take over" the system, become the largest ones. The concept of inverse cascade is the discovery for which Prof. V. Zakharov received Dirac Medal in 2003, together with Robert H. Kraichnan. We will discuss the recent numerical and experimental results on observation of the inverse cascade for gravity waves and how they relate to the Wave Turbulence Theory.